Symmetric vhf source for a plasma reactor

ABSTRACT

The disclosure pertains to a capacitively coupled plasma source in which VHF power is applied through an impedance-matching coaxial resonator having a symmetrical power distribution.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.13/047,052 filed Mar. 14, 2011 entitled SYMMETRIC VHF SOURCE FOR APLASMA REACTOR by Kartik Ramaswamy, et al., which claims benefit of U.S.Provisional Application Ser. No. 61/375,370, filed Aug. 20, 2010entitled SYMMETRIC VHF SOURCE FOR A PLASMA REACTOR, by Kartik Ramaswamy,et al.

TECHNICAL FIELD

The disclosure pertains to a capactively coupled plasma source in whichVHF power is applied through an impedance-matching coaxial resonator toan overhead electrode, and in which the coaxial resonator is folded toreduce its length, and VHF power is applied from a symmetrical feederinside the folded coaxial resonator to enhance uniformity of plasmadistribution.

BACKGROUND

A capacitively coupled plasma source for processing a workpiece, such asa semiconductive wafer, has a fixed impedance match element in the formof a coaxial resonator or tuning stub through which VHF power is appliedto a discoid or cylindrically symmetrical overhead electrode. A VHFpower generator is connected to the tuning stub at a point along itsaxis at which the RF impedance matches the impedance of the VHF powergenerator. One limitation of such a structure is that the coaxial tuningstub is exceptionally long, being on the order of a half wavelength ofthe VHF generator, which may be 0.93 meters for a VHF frequency of 162MHz. Another limitation is that the plasma distribution produced by sucha source tends to be skewed, or non-uniform in an azimuthal direction.As employed herein, the terms azimuthal and radial are employed tosignify directions in a cylindrical structure that are mutuallyorthogonal: The term radial signifies a direction along a radial linewhose origin is the cylindrical axis of symmetry. The term azimuthalsignifies a direction of travel along a circumference of the cylindricalstructure. Non-uniform plasma distribution in the azimuthal directionmay be referred to as skew. Plasma distribution may be skewed because ofasymmetrical features of the plasma reactor, such as a bend in thecoaxial tuning stub, RF-feeding of the tuning stub from one side, thepresence of a slit opening in one side of the chamber wall, and thepresence of a pumping port in the floor of the chamber of the plasmareactor.

SUMMARY

A plasma reactor includes a vacuum chamber enclosure comprising aceiling and a cylindrical side wall, the ceiling comprising a centerelectrode, a dielectric support ring around the center electrode and aworkpiece support having a support surface facing the ceiling. Thereactor further comprises a coaxial resonator comprising: (a) a hollowouter conductive cylinder coaxial with the cylindrical side wall andhaving a bottom edge on the dielectric support ring, (b) a hollow centerconductive cylinder coaxial with the outer conductive cylinder andhaving a bottom edge contacting the center electrode, and (c) an annularconductor contacting a top edge of the hollow outer conductive cylinderand contacting a top edge of the hollow center conductive cylinder. Thereactor further includes a VHF power generator and a power couplercomprising: (a) an axial center conductor connected at a first end tothe VHF generator and extending through an interior of the hollow centerconductive cylinder to a second end thereof at a selected axiallocation; and (b) plural respective spoke conductors extending radiallyfrom the second end of the axial center conductor and terminating at andcontacting the center conductive cylinder in a circular plane at theselected axial location, the plural respective spoke conductors beingsymmetrically distributed.

In one embodiment, the selected axial location corresponds to animpedance presented to the power coupler matching an output impedance ofthe VHF generator.

In one embodiment, the power coupler further comprises an axial groundedouter conductor coaxial with and surrounding the axial conductor of thepower coupler, and plural respective grounded spoke outer conductorscoaxial with and around respective ones of the plural spoke conductors,the axial grounded outer conductor and the plural grounded spoke outerconductors being joined together near the second end of the axial centerconductor.

In a related embodiment, the power coupler further comprises an axialgrounded outer conductor coaxial with and surrounding the axialconductor of the power coupler, and a grounded conductive planeextending parallel to and facing the plural spoke conductors.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the exemplary embodiments of the presentinvention are attained and can be understood in detail, a moreparticular description of the invention, briefly summarized above, maybe had by reference to the embodiments thereof which are illustrated inthe appended drawings. It is to be appreciated that certain well knownprocesses are not discussed herein in order to not obscure theinvention.

FIGS. 1A and 1B depict a plasma reactor in accordance with a firstembodiment.

FIGS. 2A and 2B depict a VHF power coupler in accordance with anotherembodiment.

FIGS. 2C and 3 are cross-sectional views of the axial and spokeconductors, respectively, in the VHF power coupler of FIGS. 2A and 2B.

FIG. 4 depicts how the geometry of the resonator is defined for optimumresonance.

FIG. 5 depicts a plasma reactor in accordance with an alternativeembodiment.

FIGS. 6 and 7 depict a plasma reactor in accordance with a furtheralternative embodiment.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. It is contemplated that elements and features of oneembodiment may be beneficially incorporated in other embodiments withoutfurther recitation. It is to be noted, however, that the appendeddrawings illustrate only exemplary embodiments of this invention and aretherefore not to be considered limiting of its scope, for the inventionmay admit to other equally effective embodiments.

DETAILED DESCRIPTION

FIG. 1A depicts a plasma reactor having a capacitively coupled plasmasource. The reactor has a chamber 100 enclosed by a cylindrical sidewall 105, a ceiling 110 and a floor 115. The ceiling 110 includes a discshaped ceiling electrode 120 surrounded by a dielectric ring 125. Aworkpiece support pedestal 130 supports a workpiece such as asemiconductor wafer 135 to be processed. VHF plasma source power isapplied to the ceiling electrode 120 in a manner described below. Theceiling electrode 120 serves as a gas distribution showerhead, havinggas injection orifices 140 in its bottom surface 120 a facing theinterior of the chamber 100. The orifices 140 may be grouped intoradially inner and outer gas injection zones 140 a, 140 b that aresupplied by gas from internal inner and outer gas manifolds 145 a, 145 bwithin the ceiling electrode 120. Internal coolant passages 150 withinthe ceiling electrode 120 facilitate heat exchange with the ceilingelectrode 120. The reactor further includes inner and outer plasmasteering magnets or coil windings 155, 160. An RF bias power generator165 is coupled through an RF impedance match element 170 to an internalelectrode 175 within the workpiece support pedestal 130.

Plasma source power is applied to the ceiling electrode 120 from a VHFpower generator 190 through a fixed impedance matching element. Incertain embodiments described herein, the fixed impedance match elementembodies a folded coaxial resonator 195, whose length is about half thatof the basic coaxial tuning stub mentioned previously herein. Thisprovides an advantageous reduction in size and increased access andserviceability. Specifically, the physical length of the folded coaxialresonator 195 is about a quarter wavelength at the frequency of the VHFgenerator 190.

The folded coaxial resonator 195 includes an inner conductive hollowcylinder 200 that is coaxial with the ceiling electrode 120. The innerconductive hollow cylinder 200 has a circular bottom edge 200 aelectrically contacting the top surface of the ceiling electrode 120.The folded coaxial resonator 195 further includes an outer hollowconductive cylinder 205 having a circular bottom edge 205 a contactingthe top surface of the dielectric ring 125. The inner and outer hollowconductive cylinders 200, 205 are of at least approximately the sameaxial length, so that their circular top edges 200 b, 205 b are at thesame height above the ceiling electrode 120. The folded coaxialresonator 195 also includes a planar conductive annulus 210 resting uponand electrically connecting the circular top edges 200 b, 205 b of theinner and outer hollow conductive cylinders 200, 205. The folded coaxialresonator 195 further includes a center hollow conductive cylinder 215coaxial with the inner and outer hollow conductive cylinders 200, 205and located between them. Preferably, the radius of the center hollowconductive cylinder 215 may be the geometric mean of the radii of theinner and outer hollow conductive cylinders 200, 205. The center hollowconductive cylinder 215 has a circular bottom edge 215 a resting on andin electrical contact with the top surface of the ceiling electrode 120.

A VHF power coupler 220 conducts VHF power from the VHF generator 190 tothe center hollow conductive cylinder 215. Thus, the center hollowconductive cylinder 215 is the RF hot center conductor of the foldedcoaxial resonator 195, while the inner and outer hollow conductivecylinders 200, 205 together with the planar conductive annulus 210 areanalogous to a grounded outer conductor of a simple coaxial resonator.The electrical connection of the circular bottom edges 200 a, 215 a tothe ceiling electrode 120 provides the requisite D.C. short, equivalentto the D.C. short at the end of a simple (unfolded) coaxial tuning stub.

As shown in FIG. 1A, the VHF power coupler 220 includes an axialconductor 222 extending through a top portion of the hollow innercylinder 200 from a top end 222 a outside of the inner cylinder 200 toan axial conductor bottom end 222 b inside of the inner cylinder 200. Afirst spoke conductor 224 a extends radially from the axial conductorbottom end 222 b through a hole 226 a in the inner cylinder 200 to thecenter conductive cylinder 215. As depicted in FIG. 1B, there are aplurality of spoke conductors 224 a, 224 b, 224 c, symmetricallyarranged and extending radially from the axial conductor bottom end 222b, through respective holes 226 a, 226 b, 226 c in the inner cylinder200 and to the center conductive cylinder 215 to which their outer endsare electrically connected. In the embodiment depicted in FIGS. 1A and1B, there are three spoke conductors 224 disposed at 120 degreeintervals, although any suitable number n of spoke conductors 224 may beprovided at 360/n degree intervals.

Conduits and conductors for various utilities extend through the hollowinner cylinder 200. These include electric conductors 250, 252 carryingD.C. current to the plasma steering magnet 155, gas lines 254, 256carrying process gas to the inner and outer gas manifolds 145 a, 145 b,and coolant conduits 258, 260 carrying coolant to and from the internalcoolant passages 150. The interior of the inner cylinder 200 is devoidof electric fields, so that arcing or electrical breakdown of theseutility lines is avoided or minimized.

In preferred embodiments, the VHF power coupler 220 is provided as acoaxial structure in which the axial conductor 222 and each of the spokeconductors 224 is a coaxial transmission line including a centerconductor that is RF hot, surrounded by a grounded outer conductor orshield. This coaxial structure is depicted in FIGS. 2A and 2B, and iscompatible with the field-free environment of the interior of the innerhollow conductive cylinder 200. In the embodiment of FIGS. 2A and 2B,the axial conductor 222 consists of a center axial conductor 222-1connected to the output of the VHF generator 190, and a grounded outeraxial conductor 222-2 surrounding the center axial conductor. FIG. 2Cdepicts a cross-sectional view of the axial conductor 222.

Each of the spoke conductors 224 a, 224 b, 224 c embodies a coaxialtransmission line structure. Thus, the spoke conductor 224 a consists ofa center spoke conductor 224 a-1 and an outer spoke conductor 224 a-2surrounding the center spoke conductor 224 a-1. The center spokeconductor 224 a-1 extends radially from the axial center conductor 222-1and terminates at and is electrically connected to the center conductivecylinder 215. The center spoke conductor 224 a-1 is RF hot by reason ofits connection to the axial center conductor 222-1. The outer spokeconductor 224 a-2 extends from the grounded axial outer conductor 222-2and is terminated at (and electrically connected to) the inner cylinder200. The center spoke conductor 224 a-1 passes through the hole 226 a(without contacting the inner conductive cylinder 200) to contact thecenter conductive cylinder 215.

The structure of each of the spoke conductors 224 a, 224 b, 224 c is thesame. Thus, the spoke conductor 224 b consists of a center spokeconductor 224 b-1 and an outer spoke conductor 224 b-2 surrounding thecenter spoke conductor 224 b-1. The center spoke conductor 224 b-1extends radially from the axial center conductor 222-1 and terminates atthe center conductive cylinder 215. The center spoke conductor 224 b-1is RF hot by reason of its connection to the axial center conductor222-1. The outer spoke conductor 224 b-2 extends from the grounded axialouter conductor 222-2 and is terminated at (and electrically connectedto) the inner cylinder 200, while the center spoke conductor 224 b-1passes through the hole 226 b (without contacting the inner conductivecylinder 200) to contact the center conductive cylinder 215.

In like manner, the spoke conductor 224 c consists of a center spokeconductor 224 c-1 and an outer spoke conductor 224 c-2 surrounding thecenter spoke conductor 224 c-1. The center spoke conductor 224 c-1extends radially from the axial center conductive conductor 222-1 andterminates at the center cylinder 215. The center spoke conductor 224c-1 is RF hot by reason of its connection to the axial center conductor222-1. The outer spoke conductor 224 c-2 extends from the grounded axialouter conductor 222-2 and is terminated at (and electrically connectedto) the inner cylinder 200, while the center spoke conductor 224 c-1passes through the hole 226 c (without contacting the inner conductivecylinder 200) to contact the center conductive cylinder 215.

The plural center spoke conductors 224 a-1, 224 b-1 and 224 c-1 extendin the radial direction from the axial center conductor 222-1 toelectrically contact the center conductive cylinder 215. The area ofthis contact defines a circular plane. The axial location of thiscircular plane is selected to be such that the electrical or RFimpedance at this location matches the characteristic impedance of 224a, 224 b and 224 c, respectively, at the frequency of the VHF generator190. The characteristic impedance of the individual spoke conductors 224a, 224 b and 224 c is selected such that their total impedance at thejunction (the axial conductor bottom end 222 b) matches the outputimpedance of the VHF generator 190 at the frequency of the VHF generator190.

Preferably, the axial center conductor 222-1 has a radius r1 that issufficient to enable the axial center conductor to carry a very high VHFcurrent, typical of current at thousands of Watts of VHF power. Forexample, as depicted in FIG. 2C, the center conductor radius r1 may beon the order of a quarter inch (or more). The axial outer conductor222-2 has an annular cross section with an inner radius r2. Thecharacteristic (transmission line) impedance of the axial conductor 222is determined by the ratio between r1 and r2 (in accordance with awell-known formula). This ratio is selected so that the transmissionline characteristic impedance of the axial conductor 222 matches theoutput impedance of the VHF generator 190.

FIG. 3 is a cross-sectional view of the coaxial transmission linestructure of the spoke conductor 224 a, and is typical of the otherspoke conductors 224 b, 224 c. The center spoke conductor 224 a-1 has acircular cross-section of radius R1, while the outer spoke conductor 224a-2 has an annular cross-section of inner radius R2. Preferably, thecenter spoke conductor radius R1 is sufficiently large to enable thespoke conductor 224 a to carry a very high VHF current, typical ofcurrent at thousands of Watts of VHF power. For example the radius R1may be on the order of a quarter inch (or more). The characteristicimpedance of the coaxial spoke conductor 224 a is determined by theratio between R1 and R2. This ratio is selected so that thecharacteristic impedance of each spoke conductor 224 is n times theimpedance of the VHF generator, where n is an integer and is the numberof spoke conductors 224. This feature ensures that the overall impedancepresented by combination of the n parallel spoke conductors 224 is thesame as the VHF generator impedance. (In the illustrated embodiments,n=3.) The structure of all of the other spoke conductors 224 b, 224 c issimilar to that of the spoke conductor 224 a depicted in FIG. 3.

In order for the folded coaxial resonator 195 to attain at leastnear-resonance at the frequency of the VHF generator 190, the electricalpath length along the interior surface formed by the inner and outerhollow conductive cylinders 200, 205 and the conductive annulus 210 isan integral fraction of the wavelength of the VHF generator. Preferably,this fraction is one-half. Thus, as depicted in FIG. 4, the sum of theaxial lengths a and c along the interior surfaces of the inner and outerhollow conductive cylinders 200, 205, respectively, and the radiallength b along the interior surface of the conductive annulus 210 isequal to the desired fraction (preferably, one half) of the VHFgenerator wavelength. However, exact resonance is not always attained bythis arrangement of the lengths a, b, and c, the discrepancy arisingfrom stray capacitances attributable to the folded geometry of theresonator. Such stray capacitances are compensated in order to attainnearly exact resonance in the folded coaxial resonator 195 by adjustingthe gap distance d between the circular top edge 215 b of the centerhollow conductive cylinder 215 and the conductive annulus 210. Thisadjustment is readily performed by the skilled worker by trial and errorto optimize resonance by the folded coaxial resonator 195 at the VHFgenerator frequency. For example, a network analyzer may be employed inconventional fashion to monitor resonance while the distance d ischanged by modifying the center hollow conductive cylinder 215.

FIG. 5 depicts an embodiment of the VHF power coupler 220 in which theaxial outer conductor 222-2 terminates at an annular ground planeconductor 300 having a circular outer edge 305 connected to the innerconductive cylinder 200. The annular ground plane conductor 300 mayinclude a bell-shaped center section 310. In the embodiment of FIG. 5,the annular ground plane conductor 300 may replace the grounded outercoaxial conductors 224 a-2, 224 b-2, 224 c-2 of the spoke conductors 224a, 224 b, 224 c.

The VHF power coupler 220 with multiple spoke conductors 224 may beemployed in cases where the resonator is not folded. FIGS. 6 and 7depict such a case, in which a simple (unfolded) coaxial resonator (orcoaxial tuning stub) 400 is a fixed impedance match element connectedbetween the VHF generator 190 and the ceiling electrode 120. The coaxialresonator 400 consists of a center hollow conductive cylinder 215, aouter hollow conductive cylinder 205, and a conductive annulus 210providing a D.C. short between the center and outer hollow conductivecylinders 215, 205. The VHF power coupler 220 in the embodiment of FIG.6 includes an axial conductor 222 extending into and coaxial with theinterior of the center conductive cylinder 215. The axial conductor 222has a center conductor 222-1 and an outer conductor 222-2 coaxial withthe center conductor 222-1. The axial center conductor 222-1 isconnected at an exterior end to the VHF power generator 190. An oppositeend of the axial center conductor 222-1, which may be referred to as theinterior end, lies within the center hollow conductive cylinder 215 at aselected axial location. The axial outer conductor 222-2 is terminatednear the selected axial location of the interior end of the axial centerconductor.

Each of the spoke conductors 224 a, 224 b, 224 c, 224 d may have acoaxial structure including center spoke conductors 224 a-1, 224 b-1,224 c-1, 224 d-1, respectively, surrounded by outer spoke conductors 224a-2, 224 b-2, 224 c-2, 224 d-2, respectively. The interior end of theaxial center conductor 222-1 provides a common terminal to which theplural center spoke conductors 224 a-1, 224 b-1, 2242 c-1, 224 d-1 areconnected. The plural center spoke conductors 224 a-1, 224 b-1, 224 c-1,224 d-1 extend in the radial direction from the axial center conductor222-1 to electrically contact the center conductive cylinder 215. Thearea of this contact defines a circular plane. The axial location ofthis plane is selected to be such that the electrical or RF impedance atthis location matches the impedance of the VHF generator 190.

As in embodiments previously described here, the ratio between the radiiof the inner and outer conductors of the axial conductor 222 is selectedto be such that the axial conductor 222 has a characteristic impedancematching that of the VHF generator. The ratios between the inner andouter conductors of each of the spoke conductors 224 is selected to besuch that the characteristic impedance of each spoke conductor 224 is ntimes the VHF generator output impedance, where n is the number of spokeconductors 224. In the embodiment of FIGS. 6 and 7, n=4.

The outer spoke conductors 224 a-2, 2224 b-2, 224 c-2, 224 d-2 extendfrom the axial outer conductor 222-2 and form a grounded conductiveenclosure. The outer spoke conductors 224 a-2, 2224 b-2, 224 c-2, 224d-2 are terminated slightly away from the interior surface of the centerhollow conductive cylinder 215 so as to not electrically contact thecenter conductive hollow cylinder 215.

The plural spoke conductors 224 a through 224 d provide an azimuthallysymmetrical distribution of VHF power to the center hollow conductivecylinder 215. The result is a more symmetrical distribution of VHFplasma source power on the ceiling electrode 120 and therefore a moreuniform distribution of plasma ion density over the workpiece supportpedestal 130.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

What is claimed is:
 1. A plasma reactor comprising: a vacuum chamberenclosure comprising a ceiling and a cylindrical side wall, said ceilingcomprising a center electrode and a dielectric support ring around saidcenter electrode; a workpiece support having a support surface facingsaid ceiling; a coaxial resonator comprising: (a) a hollow outerconductive cylinder coaxial with said cylindrical side wall and having abottom edge on said dielectric support ring; (b) a hollow centerconductive cylinder coaxial with said outer conductive cylinder andhaving a bottom edge contacting said center electrode; (c) an annularconductor contacting a top edge of said hollow outer conductive cylinderand contacting a top edge of said hollow center conductive cylinder; aVHF power generator; a power coupler comprising: an axial centerconductor connected at a first end to said VHF generator and extendingthrough an interior of said hollow center conductive cylinder to asecond end thereof at a selected axial location; and plural respectivespoke conductors extending radially from said second end of said axialcenter conductor and terminating at and contacting said centerconductive cylinder in a circular plane at said selected axial location,said plural respective spoke conductors being symmetrically distributed.2. The reactor of claim 1 wherein said selected axial locationcorresponds to an impedance presented to said power coupler matching anoutput impedance of said VHF generator.
 3. The reactor of claim 1wherein said power coupler further comprises: an axial grounded outerconductor coaxial with and surrounding said axial conductor of saidpower coupler; and plural respective grounded spoke outer conductorscoaxial with and around respective ones of said plural spoke conductors,said axial grounded outer conductor and said plural grounded spoke outerconductors being joined together near said second end of said axialcenter conductor.
 4. The reactor of claim 1 wherein said power couplerfurther comprises: an axial grounded outer conductor coaxial with andsurrounding said axial conductor of said power coupler; and a groundedconductive plane extending parallel to and facing said plural spokeconductors.
 5. A plasma reactor comprising: a vacuum chamber enclosureand a center electrode; a coaxial resonator comprising: (a) a hollowinner conductive cylinder coaxial with said center electrode and havinga bottom edge contacting said center electrode; (b) a hollow outerconductive cylinder coaxial with and surrounding said inner conductivecylinder and having a bottom edge insulated from said center electrode,said inner and outer conductive cylinders having respective circular topedges; (c) an annular conductor extending between and electricallycontacting said respective circular top edges of said inner and outerconductive cylinders; (d) a hollow center conductive cylinder coaxialwith said inner and outer conductive cylinders and located between saidinner and outer conductive cylinders, and having a bottom edgecontacting said center electrode, said center conductive cylinder havinga top edge facing and spaced from said annular conductor by an axial gaplength; a VHF power generator coupled to said center conductivecylinder; wherein said VHF generator is coupled to said centerconductive cylinder by a power coupler extending from said VHF powergenerator to said center conductive cylinder, wherein said power couplercomprises: an axial center conductor connected at a first end to saidVHF generator and extending through an interior of said hollow innerconductive cylinder to a second end thereof at a selected axiallocation; plural respective openings through said inner conductivecylinder and coinciding with a circular plane at said selected axiallocation; and plural respective spoke conductors extending radially fromsaid second end of said axial center conductor through said pluralrespective openings and terminating at and contacting said centerconductive cylinder, said plural respective spoke conductors beingsymmetrically distributed; an axial grounded outer conductor coaxialwith and surrounding said axial conductor of said power coupler; andplural respective grounded spoke outer conductors coaxial with andaround respective ones of said plural spoke conductors, and terminatingat said inner conductive cylinder, said axial grounded outer conductorand said plural grounded spoke outer conductors being joined togethernear said second end of said axial center conductor.
 6. A plasma reactorcomprising: a vacuum chamber enclosure and a center electrode; a coaxialresonator comprising: (a) a hollow inner conductive cylinder coaxialwith said center electrode and having a bottom edge contacting saidcenter electrode; (b) a hollow outer conductive cylinder coaxial withand surrounding said inner conductive cylinder and having a bottom edgeinsulated from said center electrode, said inner and outer conductivecylinders having respective circular top edges; (c) an annular conductorextending between and electrically contacting said respective circulartop edges of said inner and outer conductive cylinders; (d) a hollowcenter conductive cylinder coaxial with said inner and outer conductivecylinders and located between said inner and outer conductive cylinders,and having a bottom edge contacting said center electrode, said centerconductive cylinder having a top edge facing and spaced from saidannular conductor by an axial gap length; a VHF power generator coupledto said center conductive cylinder; wherein said VHF generator iscoupled to said center conductive cylinder by a power coupler extendingfrom said VHF power generator to said center conductive cylinder,wherein said power coupler comprises: an axial center conductorconnected at a first end to said VHF generator and extending through aninterior of said hollow inner conductive cylinder to a second endthereof at a selected axial location; plural respective openings throughsaid inner conductive cylinder and coinciding with a circular plane atsaid selected axial location; and plural respective spoke conductorsextending radially from said second end of said axial center conductorthrough said plural respective openings and terminating at andcontacting said center conductive cylinder, said plural respective spokeconductors being symmetrically distributed; an axial grounded outerconductor coaxial with and surrounding said axial conductor of saidpower coupler; and a grounded conductive plane extending parallel to andfacing said plural spoke conductors.